The invention relates generally to forced air dryers and, more particularly, pertains to solar energy-based drying of fruits, vegetables, and other food particles.
The benefits of evaporating moisture or dehydrating foods such as bananas, raisins, dates, papayas, apples, tomatoes, onions and the like are well known. Weight is reduced, making transportation and storage less expensive; food may be preserved in varying temperature environments for long periods without refrigeration and continue to provide a concentrated, pleasing flavor.
The development of dehydrating procedures includes traditional methods wherein food particles are laid out in the sun on trays for several days during which period they were exposed to dust and were frequently discolored by the sun's ultraviolet rays. The moisture content is reduced by evaporation from the initial value of 80% to the range of 15%-25%. Despite the concentrations in sweetness that accrued as a result of direct sun drying, objections arose to the products as a result of their darkened color and the dust and dirt they accumulated. Some consumers also discovered the caramelized quality of fruit sugars that are generated to a small extent in some products to be objectionable, and this caramelization was proven to be due to prolonged overheating in the sun.
Next, enclosed kilns were developed which blow heated dry air at about 65.degree. F. over and between the food particles laid out on a perforated kiln floor. Further drying compensation had to be considered when the food particles were dipped in chemical preservatives which increased their moisture content.
Another type of drying system is a continuous through circulation dryer having a natural gas burner which discharges its hot combustion into a mixture of recycled vapors from a drying bed and makeup air from the outside; the resulting hot gas and vapor mixture is drawn with a fan from a gas mixing plenum and blown into drying contact through a gas permeable bed of goods to be dried. The gas is withdrawn from drying contact and most of it is recycled for reheating while a portion is expended to the atmosphere as spent gas.
Notwithstanding these previous designs, it remains desirable to provide an improved drying system of a modified type from that known in the prior art which is effective to maintain a constant temperature range in a drying compartment to ensure even drying of food particles disposed therein. It is further desirable to provide a drying system which is solar energy-based yet contemplates the assistance of a backup fuel source for continuous 24 hour production. It is also desirable to provide a solar-based drying system which employs a collector device which permits maximum dehydrating heat flow to the food material to be dried.